1. Field of the Invention
The present invention relates to copper paste for an internal conductor of a multilayer ceramic electronic component such as a multilayer ceramic capacitor or a multilayer circuit board.
2. Description of the Background Art
FIG. 3 is a sectional view showing a conventional multilayer ceramic capacitor 1. This multilayer ceramic capacitor 1 comprises a laminate 5, which is obtained by alternately stacking a plurality of ceramic layers 2 and plural internal electrodes 3 and 4. This laminate 5 is provided with external electrodes 6 and 7 on its end surfaces respectively. The internal electrodes 3 extend toward the left end surface of the laminate 5 appearing in FIG. 3, to be electrically connected with the external electrode 6 on this end surface. On the other hand, the internal electrodes 4 extend toward the right end surface of the laminate 5 appearing in FIG. 3, to be electrically connected with the external electrode 7 on this end surface. These internal electrodes 3 and 4 are alternately arranged.
In relation to the aforementioned multilayer ceramic capacitor 1, an attempt has been made to employ copper as a metal for forming the internal electrodes 3 and 4 and the external electrodes 6 and 7, in order to reduce the cost. In this case, at least the internal electrodes 3 and 4 must be cofired with the ceramic material contained in the ceramic layers 2. Such a cofiring step must be carried out in a reducing or neutral atmosphere, in order to prevent oxidation of the copper contained in the internal electrodes 3 and 4. Further, the ceramic layers 2 must contain a nonreducing ceramic material, so that the ceramic material is not reduced by the cofiring step carried out in such an atmosphere.
In order to manufacture the aforementioned multilayer ceramic capacitor 1 comprising the internal electrodes 3 and 4 containing copper, a plurality of ceramic green sheets are prepared and copper paste is printed on specific ones of such ceramic green sheets for forming the internal electrodes 3 or 4. Then the ceramic green sheets are stacked and pressed to each other, and then cut into proper dimensions to provide a green laminate for forming the laminate 5. Then, copper paste is applied to both end surfaces of this green laminate, to form the external electrodes 6 and 7 respectively. Then the ceramic material and the copper paste contained in the as-obtained structure are cofired in a reducing or neutral atmosphere such as a nitrogen atmosphere, for example.
The multilayer ceramic capacitor 1 is obtained in the aforementioned manner.
The aforementioned copper paste contains copper powder, which is pasted with addition of an organic vehicle and a solvent.
In the aforementioned multilayer ceramic capacitor 1, an increase in electrostatic capacitance and achievement of miniaturization are required. In a multilayer circuit board, further, high densification is also required. In order to satisfy such requirements, it is necessary to reduce the ceramic layers 2 in thickness and increase the internal electrodes 3 and 4 in number of stacking.
When the cofiring step is carried out in a nonoxidizing atmosphere as hereinabove described, however, the organic vehicle contained in the copper paste for forming the internal electrodes 3 and 4 is unsatisfactorily removed and there is an insufficient debindering effect. In the cofiring stage, therefore, voids 8 are defined between the ceramic layers 2 and the internal electrodes 3 or 4 or in the internal electrodes 3 or 4 as shown in FIG. 4, to remain in the as-obtained multilayer ceramic capacitor 1. This leads to characteristic deterioration of the multilayer ceramic capacitor 1.
Further, the material forming the ceramic layers 2 is substantially different in shrinkage factor from that forming the internal electrodes 3 and 4 and the external electrodes 6 and 7. Thus, the internal electrodes 3 and 4 and the external electrodes 6 and 7 tend to warp after the cofiring step. This results in such structural defects that the internal electrodes 3 and 4 are parted, the internal electrodes 3 and 4 are improperly connected with the external electrodes 6 and 7, and the laminate 5 itself is deformed.
In addition, electric resistance values provided by the internal electrodes 3 and 4 are increased by the aforementioned voids 8 and structural defects, to result in deterioration of electric characteristics of the multilayer ceramic capacitor 1.